Zinc coated steels are known to be very old in the art and have been widely used. Large portions of such steels are made by hot dipping in a molten zinc bath at a temperature of 850.degree. F. (450.degree. C.). The zinc acts as a barrier between the substrate of steel and the atmosphere. Zinc acts as the galvanic protector sacrificing itself in the presence of corrosive elements. A plastic coating further protects the zinc.
A. J. Raymond in IRON & STEEL ENGINEER, p. 31, July 1976, describes the current manner in which steel is taken off a coil reel, is attached with a strip joiner (a welding device), is passed through a looper, a strip washer and a pinch roll unit, and then is moved to the tube forming and welding unit; for example, a Yoder mill. The steel is then cleaned and pickled, preheated to temperature, galvanized water quenched, sized and cut off. The process produces electrical conduit, EMT and I.M.C., fence tubing and galvanized mechanical tubing. Additionally, other techniques are known to exist from the following U.S. patents:
Mailhiot et al U.S. Pat. No. 3,559,280 PA1 Searing U.S. Pat. No. 3,524,245 PA1 Pierson et al. U.S. Pat. No. 4,155,235 PA1 Harris U.S. Pat. No. 3,082,119 PA1 Brick U.S. Pat. No. 3,123,493 PA1 Voss U.S. Pat. No. 3,338,208 PA1 Rogove et al. U.S. Pat. No. 4,124,932 PA1 Shoemaker U.S. Pat. No. 3,860,438 PA1 Matsudo et al. U.S. Pat. No. 3,536,036 PA1 Cleary et al. U.S. Pat. No. 3,782,909 PA1 Nakamura U.S. Pat. No. 3,927,816 PA1 Rossi et al. U.S. Pat. No. 3,845,540 PA1 Borzillo et al. U.S. Pat. No. 3,343,930 PA1 Borzillo et al. U.S. Pat. No. 3,393,089
In current practice it is possible to obtain good rust resistant coatings on sheet and tubing; however, zinc reacts with steel to form intermetallic compounds that grow rapidly in thickness at temperatures above the melting point of zinc, and the layers formed become intensely brittle. It is essential to keep such layers as thin as possible, which is precisely what the present invention intends to facilitate.
Current practice also provides for sizing after galvanizing. Many galvanized products are galvanized after manufacture is complete due to the brittle layer that has formed. However, this present process insures that the protective layer of zinc formed over the zinc/steel alloy during hot-dip galvanizing remains unbroken and undamaged by the manufacturing process.
Spangles formed on zinc create still another problem, but the present process produces a bright shiny finish that is spangle free. It is known, of course, that Canadian Pat. No. 743,047 does make an attempt to produce a spangle-free, hot-dip galvanized product and one that has a bright and shiny lustrous appearance. However, that patent works on the idea of retarding or slowing down the cooling rate of the strip by holding the strip upon emergence from the coating bath at a temperature slightly above the melting point of the coating and for a predetermined time and temperature. In the present process, the product is held at about 750.degree. F.-850.degree. F. for a period of only 5 to 30 seconds, after which the coating is allowed to solidify in the conventional manner.
METAL FINISHING magazine of September 1978 discusses other teachings which provide for hot-dip aluminum zinc alloy coatings requiring that they be hot dipped for 10 seconds in the bath, removed slowly and cooled in a blast of air. For example, Searing U.S. Pat. No. 3,524,245, cited above, teaches a sprayed zinc process, while Pierson et al. U.S. Pat. No. 4,155,235, cited above, employs a vertical air quench to solidify the aluminum coating on the tube before water quenching.
It has not been known previously to produce spangle-free galvanizing in a combination of air quenching and water quenching to preserve the brightness of the zinc coating. Rogove U.S. Pat. No. 4,124,932, cited above, refers to a pre-quenching cooling step for galvanizing tube, whereby the tubing is cooled from a flow of water and results in the surface of the zinc coating being superficially set. The tubing afterwards enters a liquid cooling bath where solidification of the zinc coating is completed.
Borzillo et al. U.S. Pat. No. 3,393,089 teach the idea of air quenching galvanized zinc, while Borzillo et al. U.S. Pat. No. 3,343,930 and Cleary et al. U.S. Pat. No. 3,782,909 teach processes for aluminum zinc coatings (all cited above). In general, however, the brittle intermetallic compound has created problems in the prior art. Cleary et al. U.S. Pat. No. 3,782,909 teach the idea of passing the coated strip through gas wiping dies (16) and of an accelerated cooling chamber (19); however, the strip is not water quenched or plastic coated. Shoemaker U.S. Pat. No. 3,860,438 is another air cooling patent as is Searing U.S. Pat. No. 3,524,245 where a Ransburg type of zinc coating is provided, and the tube is sized before coating. The metallurgy of zinc coating is covered in detail in INTERNATIONAL METAL REVIEW, Review 237, p. 1, J. Mackowiak, N. R. Short, "Metallurgy of Galvanized Coatings" (1979).
One particularly dramatic weakness of the prior art is that the sizing to final tolerance is usually accomplished after galvanizing, resulting in rupture of the bonding of the zinc to the steel. In the process of this invention the sizing is accomplished before galvanizing.